Composite materials are employed throughout industry in the manufacture of a wide variety of products ranging from automotive exterior body panels to high-cost, high-performance advanced composites, used for example in aerospace applications. Commonly, sheet molding compounds, referred to as SMCs, are used to prepare composite products, the compounds taking the form of composite sheets ready to be used for fabricating finished products, particularly those of the compression molded type. SMCs are made by combining a thermosetting resin matrix with reinforcing materials, and sometimes other modifiers, to form the sheets, such compounds having been used for many years to form low-cost, non-critical structures.
The matrices employed in preparing SMCs commonly are selected from resins such as polyesters, epoxies, vinyl esters, phenolics and the like, while the included reinforcing materials may take the form of glass, carbon and other fibers, reinforcing fillers, and combinations of the preceding, depending upon the properties required of the molded products ultimately to be formed.
Although sheet molding compounds have been widely employed, they have primarily been considered as being useful for preparing low-cost, non-critical structures. However, particularly with the advent of the space age, needs have developed for laminated and molded products capable of functioning in harsh environments, for example, where operating temperatures commonly range up to 600.degree. F. While a few exotic resins have been developed that can withstand such conditions, these have often been expensive, as well as difficult to process.
In the relatively recent past, however, a family of thermosetting polyimides has been developed whose members are not only able to function successfully in the environments described, but which are easy to process and are cost effective. These materials and the processes for preparing them, for example, are described more particularly in U.S. Pat. Nos. 3,528,950 and 3,745,149, the teachings of which are incorporated herein by reference.
The polyimide materials referred to exhibit low friction, high wear resistance, low creep, and good dimensional stability; consequently, they lend themselves to a variety of applications including the fabrication of self-lubricated parts, and for high temperature uses such as in engine parts and aircraft brakes.
Polyimides, possessing the notable properties described, have, for instance, heretofore been used in the preparation of bulk molding compounds, i.e., particulate polyimides containing reinforcing fibers usually up to about 1/4 inch in length. While such compounds lend themselves to molding intricately shaped products, unfortunately, the shortness of the fibers providing the reinforcement detrimentally affects structural strength, thus limiting the use of the products made therefrom to non-structural applications.
The polyimides have also been used in the form of laminates utilizing continuous fiber reinforcements, including those of the unidirectional, woven, knit, or braided variety. While these reinforced polyimide structures have greatly superior mechanical properties compared to the bulk molding compounds, they tend to be expensive and to be impractical for molding small and intricately shaped objects since it is difficult to conform the laminates to the surfaces of the objects.
While the use of such polyimides in conjunction with reinforcing filamentary material longer than the short fibers of the bulk molding compounds described, but shorter than the continuous fibers referred to, would be highly desirable in forming sheet molding compounds, the fabrication of satisfactory SMCs from such longer fibers is not easily accomplished. In this regard, the reinforcing fibers must be combined with a polyimide precursor solution having a viscosity that assures thorough wetting of the fibers by the solution. This is required so that satisfactory bonding of the fibers to the polyimide resin subsequently formed can be achieved. On the other hand, the viscosity must not be so low that the solution is forced out of the molding compound sheet during the process of its formation.
Furthermore, the sheet must have sufficient adhesiveness of "tack" so that the carrier film layer covering each side of the sheet will remain in place, and also so that layers of the sheets are able to adhere to each other during the process of conforming them to the molds, i.e., the "lay-up" process.
In addition, the sheets must possess sufficient "draping" ability, that is pliability, to allow them to be conformed to the surfaces of the mold.
Also, prior to being cross-linked, and after being imidized and melted, the then flowable polyimides must possess the ability to carry the contained fibers along as the resin/fiber mixture flows under pressure into all parts of the molds prior to being cross-linked. This is important since satisfactory mechanical properties, i.e., strength and stiffness, are dependent upon uniform dispersal of the fibers within the resin.
Unfortunately, some of the above-described properties are imcompatible with each other under ordinary processing conditions. For example, while good wetting is facilitated by a dilute polyimide precursor solution, a dilute solution tends to cause the precursors to be expelled by the pressures involved in formation of the sheets, and to interfere with proper tack and drape of the sheets. Conversely, if too little solvent is present, the sheets are too dry, causing them to lack the necessary tack and draping qualities.